A double row rolling bearing unit rotatably supports, for example, a wheel relative to a suspension system of a vehicle. In order to ensure running stability of the vehicle, vehicle running state stabilizing systems such as anti-lock brake system (ABS), traction control system (TCS) and electronic stability control (ESC, e.g. VSC) are widely being used. According to the running state stabilizing systems such as ABS, TCS and ESC (VSC), the running state of the vehicle at the time of braking or acceleration can be stabilized. However, in order to ensure the running stability of the vehicle under severer conditions, brakes and an engine needs to be controlled by taking in more information which influences the running stability of the vehicle.
The conventional running state stabilizing systems such as ABS, TCS and ESC (VSC) control the brakes and the engine in accordance with a detection of a slippage between the tires and the road surface, i.e., execute so-called feedback controls. Consequently, the control of the brakes and the engine is slightly delayed. In other words, for the purpose of improving performance under severe conditions, they cannot execute so-called feedforward controls to eliminate the possibility of generation of slippage between the tires and the road surface or to prevent a so-called one-side braking where braking force becomes extremely different between the right and left wheels.
In order to execute feedforward controls, a load measuring device for measuring an axial load applied to a wheel may be incorporated in a double row rolling bearing unit which supports the wheel with respect to a suspension system.
For example, JP 3-209016 A discloses a double row rolling bearing unit with a load measuring device in which load sensors are arranged at a plurality of locations on an inner surface of a fixed side flange provided on an outer circumferential surface of an outer ring, i.e., at portions surrounding screw holes into which bolts are respectively screwed to join the fixed side flange to a knuckle. In a state in which the outer ring is fixedly supported on the knuckle, each of the load sensors are held between an outer surface of the knuckle and the inner surface of the fixed side flange. An axial load applied between the wheel and the knuckle is measured by the respective load sensors.
JP 2004-3918 A discloses a double row rolling bearing unit with an axial load measuring device including displacement sensor units which are supported at four locations along a circumference of an outer ring and a detectable ring having an L-shaped cross section which is fixedly fitted on a hub. Displacements of the hub in a radial direction and an axial direction relative to the outer ring are detected at the four locations by the displacement sensor units and the detected ring, and based on the detected values at the respective locations, a direction and an amount of a load applied on the hub are obtained.
JP 62-3365 B2 discloses a method including providing a strain gauge, on a member corresponding to an outer ring whose rigidity is partially reduced, for detecting a dynamic strain, obtaining a revolution speed of rolling elements from a passing frequency of the rolling elements detected by the strain gauge, and measuring an axial load applied to a rolling bearing from the revolution speed.
Further, JP 2005-31063 A discloses a rolling bearing unit with a load measuring device in which rolling elements are arranged in double rows with mutually opposite contact angles given thereto, and a radial load or an axial load applied between a stationary bearing ring, such as an outer ring, and a rotary bearing ring, such a hub, is measured based on revolution speeds of the respective rows of rolling elements.
Meanwhile, in the double row rolling bearing units being the target of obtaining of an axial load or a radial load applied between the stationary bearing ring member and the rotary bearing ring member as described above, a preload is applied to the rolling elements thereof. The preload is applied to increase support rigidity of the rotary member such as the wheel and to enhance rotation accuracy of the rotary member.
In addition, in order to bring out sufficient performance of the double row rolling bearing unit, it is important to control the value of the preload to be within an appropriate range. If the value of the preload becomes insufficient, rigidity of the rolling bearing unit becomes low, whereby mechanical devices become likely to vibrate during operation. On the contrary, if the value of the preload becomes excessive, a surface pressure at rolling contact portions inside the double row rolling bearing unit becomes excessive, whereby the dynamic torque of the double row rolling bearing unit becomes large. As a result, not only is the performance of the mechanical devices is deteriorated but also rolling fatigue life of respective surfaces (a raceway surface and a rolling surface) at the rolling contact portions is reduced.
Accordingly, prior to the installation of the rolling bearing unit in the rotation support portion of the machineries, the preload applied to the rolling elements of the rolling bearing unit is measured so as to impart a proper preload to the rolling elements (see, e.g., JP 2001-349327 A and JP 2002-317818 A). In such a preload measuring method, however, the preload applied to the rolling element cannot be measured for the double row rolling bearing units that are already installed in the rotation support portion of the machinery and are in use.
If the preload applied to the rolling elements remains to be the proper value, there will be no specific problems. However, there are cases where the preload that has been once applied to the rolling elements changes (decreases) during the use of long period of time. The lower the preload, the larger the variation in revolution speed of the rolling elements or the relative displacement between the stationary bearing ring and the rotary bearing ring. On the contrary, the higher the preload, the smaller the variation in revolution speed of the rolling elements or the relative displacement between the stationary bearing ring and the rotary bearing ring. Because of this, depending on the changes of the preload, there is a possibility that the load applied to the double row rolling bearing unit cannot be obtained with an accuracy required, for example, to ensure the running stability of the vehicle. From the reasons given above, it is important to accurately grasp the preload applied to the rolling elements of the double row rolling bearing unit.